Micro-circuit bridge and method



1, 1967 D. N. GRIFFIN MICRO-CIRCUIT BRIDGE AND METHOD Filed June a w, 8 mfi m m m MGJ F M% a n O 0% Y B United States Patent 3,334,205 MICRO-CIRCUIT BRIDGE AND METHOD Donald N. Griffin, Redwood City, Calif., assignor to Quantic Industries, Inc., San Carlos, Calif., 21 corporation of California Filed June 23, 1966, Ser. No. 559,913 Claims. (Cl. 200-135) This invention relates generally to bridge circuits and methods for their manufacture, and more particularly to an improved micro-circuit bridge and method useful in conjunction with electro-explosive devices such as igniters, detonators, actuators, separators, and like devices employing an electric bridge to initiate a pyrotechnic or explosive material.

,Electro-explosive devices of the type described are widely used in ordnance and space technology to initiate a variety of operations such as starting rocket engines, separating rocket parts, actuating valves, and like operations. Particularly in the missile and aero-space industries, large numbers of electrically initiated squibs, detonators, gas generators and like devices are required in any complex system. On the other hand, despite the widespread use of these devices, it is generally recognized that one of the weakest links in the chain of reliability of a successfully operative system is the device used to achieve electro-explosive initation. In this regard, reliability relates not only to the reliability of functioning when desired, but also to the sensitivity of such devices to accidental initiation by stray electric or electro-magnetic current fields.

Electro-explosive devices presently in use generally employ a bridge wire circuit for initiation. The bridge wire is usually very fine as compared to other wires in the electrical circuit (on the order of 0.001 to 0.002 inch in diameter), and is attached to the terminal ends of the lead in wires by spot welding or soldering. The bridge wire therefore must be very carefully designed (i.e., as respects its composition, diameter, and electrical resistance) to insure that it will withstand a required minimum current without damage and without initiating accidentally the explosive or pyrotechnic train. Equally important, it must be able to reliably initiate the desired operation when the specified firing current is applied.

Since the bridge wire is necessarily delicate, physically weak, and easily broken, its placement in the bridge circuit in such fashion as to insure desired mechanical strength during normal handling, while retaining the desired electrical resistance characteristics for reliable functioning, presents many problems. As a practical matter, placement of the bridge wire in the bridge wire circuit is not always reliably performed by the techniques presently employed in production facilities. Not only is the attachment in the circuit often unreliable, but the effective length and cross section of the bridge Wire may vary sufliciently from one unit to the next to cause substantial variation in the essential characteristic of electrical resistance. As a result, a successful method for forming an effective, reliable bridge circuit and micro-circuit bridge is highly to be desired. 7

In general, it is an object of the present invention to provide a highly reliable bridge circuit containing a microcircuit bridge in which the bridge component not only possesses desired structural strength but also a precisely adjusted, uniform, easily reproduced electrical resistance.

3,334,205 Patented Aug. 1, 1967 Another object of the invention is to provide a bridge circuit and micro-circuit bridge of the above character which responds immediately to a specified firing current, thereby insuring reliable, elfective operation under all conditions normally encountered in service, and which also elfectively withstands a required minimum current without damage and without any tendency toward accidental initiation of the circuit.

Another object of the invention is to provide a bridge circuit wherein the bridge component possesses a desired predetermined cross sectional area and electrical resistance.

Another object of the invention is to provide a bridge circuit containing a micro-circuit bridge which is intimately bonded to a ceramic insulator to provide optimum conditions for transfer of heat from the bridge to the insulator thereby increasing the maximum current that the bridge will withstand without accidentally initiating the associated explosive or pyrotechnic materials.

Another object of the invention is to provide a method for producing a bridge circuit and micro-circuit bridge having the characteristics above specified.

Another object is to provide a method of such character which overcomes the problems normally encountered with respect to control of electrical resistance, dimensional tolerances, and like factors, within the practical limits of manufacture on a mass production scale.

Another object is to provide a method of such character which can be carried out in conjunction with or as part of the metallizing and brazing operations customarily employed in the manufacture of conventional ceramic to metal seals in initiators and igniters.

Further objects and advantages of the invention will be apparent from the following description of exemplary embodiments, and from the drawings in which:

FIGURE 1 is a perspective view of an electro-explosive initiator incorporating a bridge circuit and micro-circuit bridge according to the present invention, with parts cut away or shown in phantom;

FIGURE 2 is an enlarged perspective view, similar to FIGURE 1, showing the end face of a ceramic body portion adapted to the support of the bridge circuit and bridge (not shown);

FIGURE 3 is a like perspective view, illustrating a technique and die parts for forming a ceramic body as illustrated in FIGURE 2;

FIGURE 4 is an enlarged view in transverse section along the line 44 of FIGURE 2, illustrating a first step in a process for forming a bridge circuit and microcircuit bridge on the face of the ceramic body shown in FIGURE 2;

FIGURE 5 is a like view along the line 5--5 of FIG- URE 2;

FIGURES 6 and 7 are views like FIGURE 4 showing further steps in the processing to form the bridge and bridge circuit;

FIGURE 8 is an enlarged view similar to FIGURE 4 showing a final step in the processing to form the bridge and bridge circuit;

FIGURE 9 is an enlarged view in top plan, illustrating the finished bridge and bridge circuit, according to the embodiment of FIGURE 1; and

FIGURE 10 is an enlarged view similar to FIGURE 9, illustrating another embodiment of the invention.

Referring to' FIGURE 1, the improved bridge circuit containing the micro-circuit bridge of the invention is generally represented at 8, and is shown in conjunction with a conventional electro-explosive initiator or squib 10. The base 12 of the squib is constructed in conventional form and is adapted to house a ceramic body 14 within which one or more lead in wires can be embedded and supported. In the apparatus illustrated, two lead in wires are employed and have terminal portions or pins 22 and 24 which protrude above the surface of a substantially flat end face 20 which is provided at the upper end of the body 14. The base 12 thus encloses all but the upper end of the ceramic body 14, and thereby provides a protective support for the body 14 and the lead in wires 16 and 18. As more fully described hereinafter, metallizing material is deposited to form the bridge circuit 8, including a necked-down portion 26 and resistance leads 28, which is embedded within and bonded to the end face 20 of the ceramic body 14. A suitable pyrotechnic or explosive charge adapted to be initiated by heating of the bridge circuit 8, or by the electric spark produced by burnout of the bridge, is contained within a housing 30 (shown in phantom outline in FIGURE 1) which may be attached to the base 12 in any convenient manner.

Ceramic headers are frequently used in igniter and other initiator designs as a means to insulate and support the lead in wires. In conventional practice, these ceramic bodies are formed of powdered ceramic material such as powdered alumina, beryllia, porcelain, or other ceramic material characterized by a relatively high thermal conductivity. The powdered material is usually molded within mold parts which establish the form of the ceramic body and provide holes for passage of the terminal pins or lead in wires. The formed body is then fused or sintered at relatively high temperatures, following which the lead in wires are inserted within the body and the entire assembly subjected to metallizing and brazing procedures to effect a hermetic seal between the body and the connector pins and the cartridge base.

As noted previously, the bridge wire in conventional electro-explosive devices is very fine and delicate and must be attached to the terminal pins by soldering or spot welding. It is therefore easily broken and subject to damage during subsequent manufacturing procedures, or in the normal handling of the device following its manufacture. The bridge wire also gives rise to problems in the bridge wire circuit because the diameter of the wire is subject to variation during its manufacture which results in variation in its electrical resistance.

In accordance with the present invention, a groove or pattern for the bridge circuit (including the micro-circuit bridge) is formed directly on the surface of the ceramic body and is thereafter filled with a predetermined amount of a metallizing material suitable for use in the bridge circuit. The unit comprising the ceramic body and the metallizing material is then heated to a temperature sufficient to cause the metallizing material to fuse and bond within the groove and thereby provide a bridge circuit and micro-circuit bridge of desired cross sectional area and electrical resistance. This general processing has the advantage that the bridge can be formed during the customary metallizing and brazing operations so that it becomes integrally bonded to both the ceramic body and the electrical connectors or terminal pins, thereby eliminating the necessity for soldering or spot welding. The integral bonding of the bridge also provides a high degree of mechanical and thermal shock resistance, and a means for optimum transference of heat away from the bridge circuit so as to prevent accidental initiation at currents below the specified firing current. Most important, the described processing permits the cross sectional area and electrical resistance of the micro-circuit bridge to be accurately predetermined in a manner not heretofore possible.

Referring to the drawings, FIGURES 2 and 3 illustrate a satisfactory procedure for forming the ceramic body 14, and more particularly for forming a groove or pattern 32 in the end face 20 of the ceramic body within which a desired bridge circuit configuration may be formed. As particularly shown in FIGURE 3, mold parts 34 and 36 are provided, with the part 34 functioning as a ram to compress powdered ceramic material within the part 36 in the formation of the desired body configuration.

In the illustrated apparatus, the ram 34 is provided with two movable cylindrical projections or pins 38 and 40 which pass through the powdered ceramic material during the formation of the body to form apertures 42 and 44 for the terminal pins 22 and 24. The ram is also provided with a projecting die member 46 which projects below the lower face 48 of the ram, and is formed in the desired bridge circuit configuration. The die member 46 specifically includes a necked-down portion 50 which is carefully controlled as to cross sectional area to thereby provide a measure of control over the electrical characteristics of the micro-circuit bridge. If desired, the die member may also include circular portions 47 for the purpose of forming depressed areas 51 surrounding the apertures for the terminal pins and communicating with the groove 32.

In the molding operation to form the body 14, the die member 46 functions in a manner similar to a piece of printing type to impress the shape of the die member in the upper surface of the ceramic material retained within the lowermost part 36. The body 14 is then fired or sintered in conventional manner by heating to a temperature near its fusion temperature, prior to further processing in accordance with the invention. During the described sintering operation the molded ceramic piece is reduced in size to a final size of the body 14, approximately of the size of the original molded piece. It further becomes very hard and dense.

Referring again to FIGURE 2, the described processing produces a molded body 14 having a groove 32 below its upper surface provided with a necked-down mic-r0 groove portion 52 of desired substantially predetermined cross section. The lead in portions of the groove 32 on either side of the necked-down portion 52 are also of substantially predetermined cross section. In a preferred embodiment, the groove 32 is formed with relatively flat downward tapering side walls 54, as more particularly shown in FIGURES 5 to 7. In like fashion, the micro groove portion 52 can be formed with downward tape-ring side walls 56, such construction having been found to provide a more easily controlled cross sectional configuration and area to the critical necked-down portion 26 of the final bridge circuit.

Following the impressing of the bridge circuit pattern or groove 32 into the ceramic body 14, formation of the bridge circuit 8 is initiated by filling the groove with the particular metallizing material to be used in the process. A satisfactory metallizing material comprises a mixture of molybdenum and/or molybdenum trioxide in powdered form with manganese and/or manganese oxide powders, in combination with other oxide materials. Thus, as illustrated in FIGURE 6, the groove 32 including the micro groove portion 52 are filled to overflowing with the metallizing material, represented at 60. The metallizing material is in the form of a viscous fluid, and can be applied to the desired areas by brushing, painting, or by other suitable procedures. In addition to moly/ manganese mixtures, various other metallizing materials can be satsifactorily employed such as powdered molybdenum, titanium, gold or platinum or their alloys, or mixtures of these materials.

Subsequent to the filling of the bridge pattern or groove 32 with metallizing material, as illustrated in FIGURE 6, the upper face 20 of the ceramic body is subjected to a burnishing or wiping action to remove excess material and to provide a substantially flush surface of the metallizing material within the groove, as represented at 62 in FIG- URE 7. This wiping action can be performed in any suitable manner, for example, by using a doctor blade, or following drying, by a grinding operation. The result is that the upper face 20 of the ceramic body is wiped clean leaving the impressed groove 32 filled with the coated film of metallizing material. Since the applied metal filling will shrink substantially within the groove 32 and its necked-down micro groove portion 52, during the sintering operation employed in metallizing, the applied coatings shown in FIGURES 6 and 7 are appreciably thicker than the final film thickness shown in FIGURE 8. However, it is an important feature of the invention that the cross section-a1 area of the applied metallizing material is accurately controlled prior to this shrinkage with result that the cross sectional area of the final metal film is controlled with a degree of accuracy not hereto-fore obtained. By this process a bridge circuit can be made having a predictable electrical resistance which in general can be produced with an accuracy of approximately plus or minus 10%.

In addition to the foregoing, a degree of control or final adjustment of the bridge circuit resistance can be obtained by mechanical trimming, that is by removing small amounts of the metallizing film deposited in the non-critical areas of bridge. Satisfactory results have been obtained, for example, by bufiing, polishing, or microsandbla-sting techniques which remove small amounts of metal from the resistance leads on either side of the necked-down micro bridge 52. Any suitable buffing, polishing, or micro-sandblasting apparatus can be employed for this purpose. By this method bridge resistance can be adjusted to a value limited only by the accuracy of the resistance measuring instrument.

As a generalized illustration of the concept and practice of the invention, a bridge circuit and micro-circuit bridge is produced according to the following procedures:

A high alumina ceramic (85 to 99.9% aluminum oxide) is poured into a mold of the type illustrated in FIGURE 3, and compressed-to form a dense nonporous body of ceramic material. In general, the higher the alumina content, the higher the strength and hardness of the material and the better its dielectric properties. Consequently, an alumina content of 98% or more is to be preferred. Following the compression molding of the alumina powder within the mold, the molded body is si-ntered at a temperature of approximately 1750 C. Since the alumina body shrinks approximately 20% during the sintering process, allowance is made in the design of the compression mold to accommodate this shrinkage. By virtue of the raised pattern engraved on the mold ram, as illustrated at 46, 47, and 50 in FIGURE 3, an equivalent pattern is impressed in the flat surface of the ceramic part, as illustrated at 54, 32, and 51 in FIGURES 2 and 5. In a typical application, the necked-down portion of the groove has a length of the order of 0.01 to 0.02 inch, a width of the order of 0.002 to 0.005 inch, and a depth of the order of 0.003 to 0.005 inch. The groove in the area of the resistance lead 54 has a length of the order of 0.10 inch, and width of the order of 0.02 inch.

The metallizing material is next applied in the form of a paint over the face 20 of the ceramic body 14 so that it completely fills the impressed bridge circuit pattern or groove 32. After drying, the excess paint is removed by scraping or sanding so that the upper surface of the metallizing material is flush with the top surface 20 of the ceramic body, for example, as illustrated in the steps shown in sequence in FIGURES -6 and 7. In this manner, the metallizing material is applied to the exact pattern impressed in the ceramic, and an accurate thickness or depth of the material is maintained within the groove 32. The ceramic body 14 and applied metallizing paint is then fired in a wet hydrogen atmosphere at a temperature sulficiently high to sinter the metallizing film onto the ceramic (approximately 1570 C. for moly/manganese films). During the metallizing operation, the paint film is also applied to other surfaces of the aluminum body as required to form ceramic-to-metal seals with the electrical leads or pins 22, 24, and to the metal base 12. In a typical application, those areas of the metallizing which are to be subsequently brazed or joined to other metal components are coated with a thin layer of a material such as nickel, copper or silver which will facilitate the subsequent brazing operations. For example, where a moly/manganese film is applied as the metallizing material, a thin layer of nickel oxide paint can be applied to such metal components, following which the part is fired at 900 to 1000 C. in a hydrogen atmosphere which reduces the nickel oxide to form a uniform nickel plate on the moly/manga-nese film. Such procedure provides a suit-able surface for brazing or soldering. Specifically, in the illustrated embodiment, the nickel coat is applied at the ends of ,the groove patterns in the areas 51 where the bridge circuit is brazed to the connector pins 22 and 24.

The ceramic body 14 is next assembled with the connector pins and with the metal base 12 and brazed in a hydrogen or inert atmosphere furnace. Either a single step or two-step brazing operation can be employed. Where a two-step operation is used, the pins represented at 22 and 24 are brazed to the part as generally indicated in FIGURE 1, using a high temperature brazing alloy, following which the ceramic subassembly consisting of the ceramic body and pins is brazed in a second operation to the metal base 12, using a lower temperature brazing alloy. In this processing, brazing temperatures vary from about 360 to 1100 0, depending upon the particular brazing alloy used. For example, one satisfactory alloy comprises 72% silver intermixed with 28% copper (M.P. 780 0.). Such alloy might be successfully used' in the first step of the two-step process. A satisfactory alloy for the second step may comprise an alloy consisting of 88% gold intermixed with 12% germanium (M.P. 356 C.). Following such brazing operation, the finished component is combined into a final assembly with the housing 30 which contains appropriate pyrotechnic materials, a typical assembly being shown in FIGURE 1.

In carrying out the present invention, a satisfactory metallizing paint can comprise a mixture of molybdenum and/ or molybdenum-trioxide in powdered form with manganese and/or manganese oxide powders, preferably combined With other oxide materials (e.g., the oxides of aluminum, barium, boron, calcium, potassium, silicon, sodium, and tantalum). Thus, assuming the use of molybdenum and manganese powders, a typical metallizing formulation might have the following composition:

Percent Molybdenum power 75 Manganese powder 12.5 Other metal oxides, either singly or in combination 12.5

The powdered metal oxides (or metals) are mixed with a suitable lacquer binder to give a paint having approximately 55 to 58% solids by weight, for example, with a lacquer binder having the following formulation:

Percent Acetone 31.4 Ethyl ether 28.6 Methyl/ethyl/ketone 14.3 Nitrocellulose lacquer (600-1000 seconds) 25.7

The lacquer binder and metallizing formulation, as above, are ball milled to provide a satisfactory dispersement of the ingredients, and then applied by brush or other suitable painting technique to the grooved area 32 and surface 20 of the ceramic body, as above described.

During the final heating or sintering operation, the metallizing material in the groove 32 shrinks somewhat, as illustrated in FIGURE 8. However, since a predetermined volume of metallizing material is contained within the impressed bridge pattern, the cross sectional area of the final metal film is accurately predetermined. The described procedure thus insures that a desired predetermined electrical resistance within the bridge circuit 8, and particularly within the micro-circuit bridge portion 26, can be accurately obtained. For example, using the above described formulation and a bridge pattern as shown in FIGURE 9, micro-circuit bridges are obtained having a resistance of the order of 0.78 ohmi0.10 ohm. In conjunction with a charge of a typical initiating explosive within the housing 30, for example, lead styphnate, the described micro-circuit bridge will reliably initiate the explosive when a current of 2.4 amperes is applied. With a current of 5.4 amperes, the explosive can be reliably initiated within 1 millisecond of the time of first application of the current. On the other hand, upon application of a current of 1.5 amperes, the micro-circuit bridge will successfully resist initiation.

The foregoing example illustrates the general practice of the invention. It will be understood, however, that the present invention is not limited to any particular formulation of a metallizing paint. In general, metallizing formulations employing approximately 50 to 90% of molybdenum and/ or molybdenum trioxide powders mixed with approximately 5 to 30% of manganese and/ or manganese oxide powders will provide satisfactory results. Other metallizing materials as herein disclosed, as well as other proportions and percentages of ingredients are also clearly within the intended scope of the invention herein disclosed and claimed.

The bridge circuits of the invention may be constructed in various forms or configurations, depending on the requirements of a particular design or specification. ,Thus, the configuration of FIGURES 1 through 9 illustrates a particular pattern or configuration of the bridge circuit as might be used in an electro-explosive device having a 2 amp/2 watt no-fire capacity, meaning that the circuit will withstand two amperes of current or two watts of power without firing. FIGURE 10 illustrates a possible variation in design to meet the present 1 amp/1 watt no-fire characteristic generally imposed on the industry. It will be understood that these and other design configurations (both in plan and cross section) are clearly within the scope of the present invention.

From the foregoing, it will be apparent that the present invention makes possible the manufacture of micro-circuit bridges which can be successfully adjusted during the manufacturing process to obtain a predetermined electrical resistance and firing characteristic related to the cross sectional area of the micro bridge. Specifically, the present invention provides a simple procedure for controlling the cross sectional area of an applied metal film which constitutes the micro bridge. In addition, a degree of control or final adjustment of the bridge circuit resistance can be obtained by mechanical trimming, that is, by removing small amounts of the metallizing film deposited in the non-critical areas of the bridge. By this method bridge resistance can be adjusted to a value limited only by the accuracy of the resistance measuring instruments.

The process of the invention also makes possible the integral bonding of the applied metal film circuit and bridge to the ceramic substrate, simultaneously with the performance of metallizing and brazing operations in which the terminal pins are brazed within the ceramic. The invention thus facilitates the mass production manufacture of systems characterized by an unusually high degree of mechanical and thermal shock resistances. The integral bonding technique also provides a finished bridge system in which a maximum transfer of heat away from the bridge circuit is obtained, thereby providing maximum protection against accidental firing of the circuit below a predetermined firing current. Finally, the invention provides a simple technique and a micro-circuit bridge having a wide applicability and utility in electro-explosive devices, a high degree of reliability to insure firing on command, and maximum safety from accidental initiation or misfire.

I claim:

1. A method for the manufacture of a micro'circuit bridge device for use in a bridge circuit, the steps of forming a substantially fiat surface on a relatively dense non-porous body of ceramic material while simultaneously impressing therein a groove having a necked-down portion of predetermined cross section, filling said groove with a metallizing material suitable for use in said bridge circuit and micro-circuit bridge so that the exposed surface of metallizing material deposited in said groove is flush with the substantially flat surface of said body, and heating said body and deposited metallizing material to a temperature sufficient to cause said metallizing material .to fuse into a unitary mass and to bond to surfaces of the groove within said body of ceramic material to thereby form a bridge circuit device and micro-circuit bridge device wherein the necked-down bridge portion has a desired predetermined cross sectional area and electrical resistance.

2. A method as in claim 1 wherein said body of ceramic material is formed by compressing a powdered ceramic material within a mold whereby said groove is engraved within said substantially fiat surface.

3. A method as in claim 2 wherein said powdered ceramic material is powdered alumina.

4. A method as in claim 3 wherein said powdered ceramic material is a mixture of ceramic materials containing from to 99.9% aluminum oxide.

5. A method as in claim 1 wherein said metallizing material consists essentially of a mixture comprising from 50 to of material selected from the group consisting of molybdenum and molybdenum trioxide, from 5 to 30% of material selected from the group consisting of manganese and manganese oxide and the remainder metal oxides selected from the group consisting of aluminum, barium, boron, calcium, potassium, silicon, sodium and tantalum oxides.

6. A method as in claim 1 wherein said groove is overfilled with said metallizing material, following which excess metallizing material is removed to provide an exposed surface of metallizing material substantially flush with the flat surface of said body.

7. A bridge circuit device, an insulating ceramic body having aperture means for lead-in wires, said ceramic body having a substantially fiat end face provided with impressed groove means, said groove means having a necked-down portion of reduced predetermined cross sectional area, said necked-down portion of the groove means having a cross sectional configuration in the form of a downward taper provided byrel-atively flat wall surfaces, metallized conductive means substantially filling and bonded to the groove means and the relatively flat wall surfaces of said necked-down portion, said metallized means constituting a conductive path in said bridge circuit with the metallized means of said necked-down portion of the groove comprising a micro-circuit bridge of predetermined cross sectional area and electrical resistance, and lead-in wire means extending through the aperture means of said body to provide electrical connections to said bridge circuit and said micro bridge.

8. A bridge circuit and micro-circuit bridge as in claim 7 wherein said ceramic body consists essentially of dense non-porous sintered powdered alumina.

9. A bridge circuit and micro-circuit bridge as in claim 7 wherein said bridge consists of a sintered metal film composed essentially of 50 to 90% molybdenum, 5 to 30% manganese, and the remainder metal oxides selected from the group consisting of aluminum, barium, boron, calcium, potassium, silicon, sodium and tantalum oxides.

10. A bridge circuit device and micro-bridge circuit device as in claim 7 wherein the metallized conductive means within said necked-down portion is at a point of face.

9 10 predetermined shrinkage below said substantially flat end 3,042,591 7/ 1962 Cado 174-68.5 3,042,741 7/1962 Cumpston 174-685 FOREIGN PATENTS References (11M 5 867,090 5/1961 Great Britain.

UNITED STATES PATENTS OTHER REFERENCES Kub 1ak 200-130 Levin et al.: Phase Diagrams for Ceramists, 1964 ed., Suulvan Columbus, Ohio. The American Ceramic Society, Inc., Massar et a1. 200-142 pp. 246 and 272, 1950, 3rd ed., London, C. Griflin. Meisel et a1. 317234 10 Treptow 33 BERNARD A. GILHEANY, Primary Examiner.

Byer et a1. 174-68.5 H. B. GILSON, Assistant Examiner. 

7. A BRIDGE CIRCUIT DEVICE, AN INSULATING CERAMIC BODY HAVING APERTURE MEANS FOR LEAD-IN WIRES, SAID CERAMIC BODY HAVING A SUBSTANTIALLY FLAT END FACE PROVIDED WITH IMPRESSED GROOVE MEANS, SAID GROOVE MEANS HAVING A NECKED-DOWN PORTION OF REDUCED PREDETERMINED CROSS SECTIONAL AREA, SAID NECK-DOWN PORTION OF THE GROOVE MEANS HAVING A CROSS SECTIONAL CONFIGURATION IN THE FORM OF A DOWNWARD TAPER PROVIDED BY RELATIVELY FLAT WALL SURFACES, METALLIZED CONDUCTIVE MEANS SUBSTANTIALLY FILLING AND BONDED TO THE GROOVE MEANS AND THE RELATIVELY FLAT WALL SURFACES OF SAID NECK-DOWN PORTION, SAID METALLIZED MEANS CONSTITUTING A CONDUCTIVE PATH IN SAID BRIDGE CIRCUIT WITH THE METALLIZED MEANS OF SAID NECK-DOWN PORTION OF THE GROOVE COMPRISING A MOCRO-CIRCUIT BRIDGE OF PREDETERMINED CROSS SECTIONAL AREA AND ELECTRICAL RESISTANCE, AND LEAD-IN WIRE MEANS EXTENDING THROUGH THE APERTURE MEANS OF SAID BODY TO PROVIDE ELECTRICAL CONNECTIONS TO SAID BRIDGE CIRCUIT AND SAID MICRO BRIDGE. 